For power transmission over distance it is usual to transmit power over transmission lines at high voltage to reduce power losses. Typically power is transferred using high voltage AC power transmission. Three phase AC transmission is the most common, although for supplying some remote areas fewer phases may be used.
On land, outside of urban areas, it is typical to transmit power using high voltage AC power distribution using overhead transmission lines. The various conducting lines are suspended so as to be sufficiently far above the ground and away from one another to provide electrical isolation, i.e. the surrounding air acts as an insulator.
In some instances however such overhead lines may not be practical or appropriate. For instance in urban environments it may be desired to transmit and distribute power using buried cables rather than overhead lines. For transmission of power across bodies of water it may also be better to use submarine power cables. This is becoming increasingly of interest with the growing popularity of offshore power generation, e.g. offshore wind farms and the like.
In a power distribution cable it is necessary to insulate the conductors that are used for power transfer from one another and also from the environment. FIG. 1 illustrates the general principles of such power cables and shows a cross-section of one example of a power cable 100 such as may be used, for example, for medium or high voltage AC power transmission, i.e. voltages of tens or possible hundreds of kilovolts.
FIG. 1 illustrates that the cable may have three conductors 101, one for each AC phase, each of which may, for example, be formed from copper or a similar material. The conductors are each sheathed by at least one semi-conductive layer 102 which is surrounded by a respective insulator 103 such as XLPE (cross-linked polyethylene). Each insulator 103 may be surrounded by one or more sheathing layers 104 and 105, which may, for example, include at least one semi-conductive sheathing layer 104. For a high voltage cable, layer 105 could be a lead sheath for example whereas for a medium voltage application layer the layer 105 may be a copper screen. It will be understood by one skilled in the art there may be additional or alternative sheathing layers such as semi-conductive polyethylene, aluminium tape, a conductive counter-helix or the like and/or other layers such as swelling tape surrounding the insulator. The sheathing layers, i.e. 102, 104 and/or 105, are arranged to provide electric shielding to shield the other conductors and the environment from any electric field generated by the current flowing in the conductors in use.
The three conductors, with their associated sheathing and insulation layers, are all contained within an armour layer 106 which may for instance comprise a braiding of galvanised steel wires to provide protection for the power cable. The power cable may also have an outer jacket layer 107 such as a polypropylene yarn cladding.
There may be filler material 108 within the cable, which may comprise a plurality of elongate filler elements disposed inward of the armour layer 106. This can give the overall power cable a desired form and ensure the sheathed conductors are held in place within the power cable, as well as providing additional padding/protection.
Additionally it is common to embed at least one optical fibre within the cable, or at least provide the ability for optical fibres to be located within the power cable, for instance to allow for data communication between the various power stations linked by the cable. Thus there may be at least one fibre optic conduit 109 for carrying one or more optical fibres 110, and typically a bundle of optical fibres.
As mentioned one particular area of application for such power cables is for carrying power across bodies of water, e.g. from an offshore power generation site, such as a wind farm, as illustrated in FIG. 2. FIG. 2 illustrates that a first power station 201, which in this example may be located on an off shore platform, may be connected to a local source of power, such as a plurality of wind turbines 202. The first power station 201 may receive electrical power from the wind turbines and in some instances transform the voltage to a high voltage for transmission to an on-shore power station 203 via power cable 100. Power cable 100 may be deployed to run along the sea-bed to shore. In examples such as this wind farm example the first power station 201 may be several kilometres or several tens of kilometres from shore.
In use the conductors of the power cable 100 may carry very high voltages, of the order of hundreds of kilovolts for example. Thus good quality insulation is required and it is important that the cable is substantially defect free.
Submarine power cables may in some instances be damaged or become highly distorted during the deployment or during subsequent use. If a cable is damaged or highly distorted this may degrade the insulation and, in use with the operating voltage applied to the cable, the insulation may fail such that there is a discharge between conductors of the cable, or between a conductor and the environment, i.e. earth. This can result in a high voltage discharge with a significant fault current, e.g. arcing. Typically such a fault may result in a catastrophic failure of the relevant part of the cable, with potentially explosive failure of the cable. FIG. 2 illustrates a cable insulation failure and catastrophic fault at location 204. This will generally result in power transmission through the cable having to be stopped until the cable can be repaired, for instance by opening one or more high voltage circuit breakers 205. It would therefore be useful to be able to check a cable for any defects after installation or any downtime prior to re-energising the cable and/or detect the location of a fault when it occurs.
In use it would also be desirable to be able to detect significant movement of the power cable, for instance resulting from vortex induced vibration of the cable or free-spanning of the power cable. Free-spanning refers to a submarine cable floating freely. Many submarine cables are buried in the sediment on the sea bed to prevent movement of the cable and provide protection. Over time however sections of the power cable may become exposed, and thus more prone to damage. FIG. 2 for example illustrates that the cable may be largely buried but a section 206 may be exposed and free spanning. Movement of an exposed cable can also lead to scour effects. Vortex induced vibration (VIV) in particular is vibration induced on the cable due to the cable interacting with flow around the cable. VIV can lead to/accelerate fatigue in the power cable.
Power cables of the type described can be very expensive components and failure of a power cable can result in power distribution via the cable having to be halted. It would therefore be beneficial to be able to detect when a cable is free-spanning and/or being subjected to VIV as it may be desirable to bury or rebury the cable to prevent damage.
Embodiments of the present thus relate to methods and apparatus for monitoring of power cables.